Much of the world's production of copper, zinc and nickel is recovered by electrolytic processes in which the metal is produced in relatively pure form by deposition from aqueous solutions, on the cathodes of electrolytic cells. Similar electrolytic processes are sometimes used for the production of cobalt, manganese and the precious metals. In the electrowinning of metals, the metal to be recovered is first dissolved by chemical means in the electrolyte, which is then passed through an electrolytic cell containing insoluble anodes. The metal ions in the electrolyte are discharged at the cathode surface by passage of direct electric current, to form a deposit of relatively pure metal, while an equivalent number of negative ions, such as hydroxyl or sulfate ions, are discharged at the anode and then reform with the release of molecular oxygen bubbles. In some cases, the reaction at the anode involves the oxidation of metal ions, such as ferrous or cuprous ions, which are converted to the ferric and cupric state, respectively. The insoluble anode material used in electrowinning operations is usually a lead or lead alloy, or graphite or titanium coated with a thin coating of platinum. Certain other anodically insoluble materials have also been used.
In the design of electrolytic cells, it is necessary to provide open access at the top surface of each cell, so that the cathodes may be easily inserted and removed and to permit the escape of oxygen gas formed at the anode and any hydrogen gas which may be produced at the cathode. The surface of the electrolyte in these cells is therefore essentially at atmospheric pressure.
Extensive research has been conducted to determine the most favorable or most economical conditions for electrolytic cell operation in specific applications. One important operating factor is the velocity of flow of the electrolyte across the active faces of the electrodes. It has been demonstrated by many investigators that it is desirable to maintain a flow of electrolyte at relatively high velocities across the faces of the electrodes. This eliminates or reduces the ion-deficient film which tends to form at these faces during electrolysis. Some specific advantages of maintaining high velocity flow of the electrolyte across the electrode faces are as follows:
(1) The cathodically deposited metal tends to form a more coherent, smooth and dense deposit relatively free from the nodule or "tree" growths, which often cause short circuiting of the electric current between anodes and cathodes. PA1 (2) Good quality cathode deposits can be formed at higher current densities, which in turn increase the metal productions rate of each cell. PA1 (3) For a given current density, the voltage drop between anode and cathode is reduced, thus saving in the electric energy required per pound of metal deposited at the cathode. PA1 (4) Cathode current efficiency is improved, since there is less evolution of hydrogen at the cathode and since there is also less short circuiting through nodule or "tree" growths on the cathode. The improvement in current efficiency also produces an equivalent improvement in energy efficiency. PA1 (5) Impurities in the cathode deposit are reduced, since the deposit is smooth and does not entrap particulates suspended in the electrolyte. PA1 (6) Since the metal-ion-deficient film at the cathode is thinner, it is possible to electrowin certain metals, with reasonable current efficiencies, from very dilute solutions of that metal. PA1 (7) Under some operating conditions, such as when very dilute solutions are electrolyzed, or when very high current densities are applied, the metal is deposited at the cathode as a loosely adhering sponge. With a high flow rate of the electrolyte across the cathode surface, this metal sponge becomes suspended in the electrolyte and can be collected, as fast as it forms, from the discharged electrolyte. A cell of this type requires little manual attention and operates with a relatively low labor cost. PA1 (8) Slurry suspensions of ground ore or concentrates can be maintained in the electrolyte. Certain metals, such as copper, lead and nickel can be concurrently leached and electrowon from these slurries to provide a relatively low cost and non-polluting method for recovery of these metals from their ores or concentrates.
The attainment of high velocity flow of the electrolyte across the electrode faces of electrowinning cells is fairly easily accomplished in laboratory size cells by mechanical stirring, by the use of a rotating anode or cathode, or by providing a rising flow of gas bubbles between the anode and cathode. Ultrasonic agitation also has proved advantageous in improving the physical quality of cathode deposits. The advantages of high velocity flow and some of the methods of cell designs developed to improve flow velocities in laboratory or commercial cells are well summarized in the paper by W. R. Hopkins, G. Eggett and J. B. Scuffham on "Electrowinning of Copper," Chapter 7, International Symposium on Hydrometallurgy, A.I.M.E. meeting, Chicago, IL, Feb. 25-Mar. 1, 1973, Library of Congress Card No. 72-88874. Their summary indicated that good cathode deposits could be produced at current densities of 100 amperes per square foot in copper electrowinning cells, when the electrolyte flow velocity between the electrodes was 15 feet per minute. For comparison, in normal copper electrolytic cells, where flow velocities are generated by convection currents of only a few feet per minute at best, it has been necessary to hold current densities down to about 15 to 20 amperes per square foot, in order to obtain cathode deposits of good physical quality. Other investigators have reported that flow velocities of about 50 feet per minute substantially reduced the thickness of the ion-deficient film on the electrode surfaces.
Commercial electrowinning cells usually involve the use of about 50 or more cathode sheets or plates uniformly spaced in interleaved arrangement with plate type anodes inside a rectangular tank. Total active surface area of the cathodes in each cell usually exceeds 1000 square feet. Heretofore, no economically practical method has been developed to maintain electrolyte flow velocities in excess of 10 feet per minute over such large cathode areas in these commercial cells. Several methods of improving flow velocities have, however, achieved limited commercial success. One such method involves the CCS cell design described in the paper by P. T. W. Strub and E. J. Clugston on "The CCS Cell Reduced to Practice," A.I.M.E. annual meeting, San Francisco, California, 1972, in which the electrolyte is recirculated at high volume rates to a central header in the bottom of each cell and is swept up between anode and cathode pairs to discharge over weirs on each side of the cell. This achieves upward flow velocities up to about 6 feet per minute, but requires recirculation pumping of very high volumes of electrolyte. The high investment and operating cost of these cells has prevented their adoption at most electrowinning plants.
Another method of improving flow velocities of the electrolyte between anode and cathode pairs is to inject gas (usually air) into the electrolyte at the bottom of the cell, so that the gas bubbles sweep upward between the electrode surfaces. This method, however, produces several disadvantages. The rising gas bubbles increase the electrical resistance and consequently the voltage drop between anode and cathode and thus reduces energy efficiency. The gas bubbles also tend to reduce the operating temperature of the electrolyte, which further increases voltage drop. Consequently, this method of "gas sparging" has received little use commercially.
Another means of increasing the velocity of electrolyte flow, by the use of deflectors appropriately spaced along each side of the cell, was proposed by W. R. Sorenson et al in U.S. Pat. No. 3,558,455. However, this method produces only a mild increase in flow velocities, which are well below the desired minimum velocities of 10 to 15 feet per minute.